1. Field of the Invention
The present invention relates to a fuel injection control apparatus for an internal combustion engine including an in-cylinder injection valve that directly injects fuel into the combustion chamber and an out-cylinder injection valve that injects fuel to the intake manifold.
2. Description of the Background Art
In controlling the fuel injection for an internal combustion engine, the value of the required quantity of fuel injection is determined according to the operating status of the engine. Fuel of the injection quantity corresponding to this required value (namely, the entire required injection quantity) is injected from the fuel injection valve. Such fuel injection valves include an in-cylinder injection valve that directly injects fuel into the combustion chamber of an internal combustion engine in addition to an injection valve (out-cylinder injection valve) that injects fuel to an intake manifold such as an intake port. When the in-cylinder injection valve is employed, conversion of injection fuel into fine particles must be facilitated since it is difficult to ensure a sufficient period of time for the injected fuel to be mixed with the incoming air to be vaporized. Therefore, an in-cylinder injection valve is adapted to increase the fuel supply pressure to inject fuel at a higher pressure, as compared to an out-cylinder injection valve.
There is proposed a fuel injection control apparatus including both an in-cylinder injection valve and an out-cylinder injection valve, wherein the injection quantity ratio of each injection valve to the entire required injection quantity at the starting time is set variable based on the fuel supply pressure of the high-pressure fuel supply system, i.e. the fuel supply pressure of the in-cylinder injection valve (Japanese Patent Laying-Open No. 2001-336439).
When the internal combustion engine is under high load such as at the time of going up a hill or during acceleration in such a conventional fuel injection control apparatus including both an in-cylinder injection valve and out-cylinder injection valve, the injection quantity ratio of the in-cylinder injection valve (required injection quantity=required fuel injection quantity) is increased to lower the temperature of the combustion chamber by the heat of vaporization from the fuel directly injected into the combustion chamber, whereby the intake efficiency, and in turn the engine output, is improved. The fuel supply pressure of the high-pressure fuel supply system is increased to conduct fuel injection of the required injection quantity in an appropriate manner.
The high-pressure fuel supply system includes a pump of high pressure and a metering valve for adjusting the fuel emission from the high-pressure pump. Noise is generated in accordance with the pump fuel pressurization and the opening/closure of the metering valve in the high-pressure fuel supply system. The noise generated from the high-pressure fuel supply system is apt to become more noticeable at the time of lower load on the engine where other noises (for example, the noise of burning fuel in the engine or the noise generated from the driving mechanism) are smaller, i.e. when the required injection quantity of the in-cylinder injection valve is lower.
A likely approach is to reduce the fuel supply pressure of the high-pressure fuel supply system, or completely cease the fuel supply, at a low load mode of the engine where the required injection quantity of the in-cylinder injection valve is low. Accordingly, the noise encountered in the unnecessary boosting of the fuel supply pressure in the high-pressure fuel supply system as well as the operating noise from the pump can be reduced at least when the load is low.
However, if the fuel supply pressure of the pump of high pressure is once reduced to lower the pressure of fuel supplied to the in-cylinder injection valve in a low load status, fuel injection by the in-cylinder injection valve will be conducted while the fuel supply pressure is not yet boosted to the sufficient level during the transition to a high load. This will be described more specifically with reference to FIG. 5. The fuel supply pressure supplied to the in-cylinder injection valve will not directly exhibit a transition from the fuel supply pressure of low level in a low load mode to the target supply pressure P0 of high level corresponding to the larger required injection quantity, as indicated by the chain with two dots in FIG. 5. A time lag t will occur between a time point t1 where the operating status corresponds to a high load and a time point t2 where the fuel supply pressure attains the level of the target supply pressure P0. In the event of such insufficient boosting in pressure, the injection duration must be extended, corresponding to the low fuel supply pressure, in order to inject fuel of a quantity equal to the required injection quantity from the in-cylinder injection valve.
In the case where the fuel supply pressure of the in-cylinder injection valve is actually greatly lower than the target supply pressure, the injection period will become longer than the injection allowable period. This means that the injected fuel will be short in supply to cause degradation in the engine output. Furthermore, a low fuel injection pressure (the same value as fuel supply pressure) will not facilitate conversion of the injection fuel into fine particle, leading to the problem of deteriorating fuel combustion.
The aforementioned publication of Japanese Patent Laying-Open No. 2001-336439 discloses control to conduct fuel injection reliably corresponding to the required injection quantity by modifying the injection quantity ratio of the in-cylinder injection valve to a low level based on the fuel supply pressure of the high-pressure fuel system. However, this only addresses the lacking injection quantity caused by the insufficient rise of the fuel supply pressure at the starting time. It does not deal with the disadvantage that occurs in a configuration that intentionally reduces the fuel supply pressure of the high-pressure fuel supply system when the operating status of the engine is shifted from a high load to a low load.